Résumé
Gene duplications have long been advocated to contribute to the evolution
of new functions. The role of selection in their early spread is more
controversial. Unless duplications are favored for a direct benefit of
increased expression, they are likely detrimental. In this paper, we
investigated the case of duplications favored because they combine already
functionally divergent alleles. Their gene-dosage/fitness relations are
poorly known, because selection may operate on both overall expression and
duplicates relative dosage. Using the well-documented case of Culex
pipiens resistance to insecticides, we compared strains with various ace-1
allele combinations, including two duplicated alleles carrying both
susceptible and resistant copies. The overall protein activity was nearly
additive, but, surprisingly, fitness correlated better with the relative
proportion of susceptible and resistant copies rather than any absolute
measure of activity. Gene dosage is thus crucial, duplications stabilizing
an ‘heterozygote’ phenotype. It corroborates the view that these were
favored because they fix a permanent heterosis, thereby solving the
irreducible trade-off between resistance and synaptic transmission.
Moreover, we showed that the contrasted successes of the two duplicated
alleles in natural populations depend on genetic changes unrelated to
ace-1, confirming the probable implication of recessive sublethal
mutations linked to structural rearrangements in some duplications.